Devices which respond to shorter wavelength incident radiation to emit radiation of a longer wavelength are known. Any device provided with a fluorescent layer will in response to exciting radiation in a certain wavelength range emit radiation in a longer wavelength range characteristic of the fluorescent material. When the exciting radiation, however, extends over a wide spectral region, then only a very narrow range of wavelengths above its absorption edge are capable of exciting the fluorescent material, and the rest of the exciting spectrum is effectively wasted. Thus, only a small part of the energy associated with the incident radiation is used. Where, for example, the desire is to excite the fluorescent material with solar radiation, then most of the solar energy is wasted and only a relatively small part is converted to the characteristic radiation of the fluorescent material.